What is the net charge on Phe-Asp at each pH?
a. pH 1
b. pH 7
c. pH 14
To determine the net charge on Phe-Asp at each pH, we need to consider the pKa values of the amino acid residues involved and their protonation states at each pH.
Phe-Asp contains two amino acid residues: phenylalanine (Phe) and aspartic acid (Asp).
1. At pH 1 (acidic conditions) with a lower pH than their pKa values:
- Phenylalanine (pKa ≈ 9.2) will be protonated, as its pKa is higher than the pH, giving it a positive charge (+1).
- Aspartic acid (pKa1 ≈ 2.0 and pKa2 ≈ 9.9) will be fully protonated, with both carboxyl groups (COOH) ionized and carrying a negative charge (-2).
Therefore, the net charge on Phe-Asp at pH 1 is (+1) + (-2) = -1.
2. At pH 7 (physiological conditions):
- Phenylalanine (pKa ≈ 9.2) will be deprotonated, as its pKa is higher than the pH, making it neutral (charge = 0).
- Aspartic acid (pKa1 ≈ 2.0 and pKa2 ≈ 9.9) will have only one carboxyl group (COO-) ionized while the other remains uncharged, resulting in a net charge of -1.
Therefore, the net charge on Phe-Asp at pH 7 is 0 + (-1) = -1.
3. At pH 14 (alkaline conditions) with a higher pH than their pKa values:
- Phenylalanine (pKa ≈ 9.2) will be deprotonated, as its pKa is higher than the pH, making it neutral (charge = 0).
- Aspartic acid (pKa1 ≈ 2.0 and pKa2 ≈ 9.9) will be fully deprotonated, with both carboxyl groups (COO-) remaining uncharged.
Therefore, the net charge on Phe-Asp at pH 14 is 0 + 0 = 0.
To determine the net charge on Phe-Asp at different pH values, we need to consider the ionization states of amino acid residues involved in the Phe-Asp peptide sequence.
At pH 1 (very acidic conditions), most of the amino acid side chains will be in their protonated or positively charged forms. Therefore, we need to check the pKa values of the acidic and basic residues present in Phe-Asp.
Phe-Asp contains the amino acid residues Phenylalanine (Phe) and Aspartic Acid (Asp). Phenylalanine is a non-ionizable or neutral amino acid, meaning it does not gain or lose protons with changes in pH. Aspartic Acid, on the other hand, has a carboxylic acid group (COOH) as its side chain. The pKa value for the carboxyl group of Aspartic Acid is around 3.9.
At pH 1, the carboxyl group of Aspartic Acid would be protonated (COOH2+), contributing a positive charge to the molecule. Therefore, the net charge on Phe-Asp at pH 1 would be +1.
At pH 7 (physiological pH), we need to consider the ionization of the residue Aspartic Acid. At pH 7, which is close to its pKa value of around 3.9, roughly 50% of the Aspartic Acid side chains will be protonated (COOH2+), and the other 50% will be deprotonated (COO- or negatively charged). Since Phe does not ionize at physiological pH, the net charge on Phe-Asp would be -0.5 (equal distribution of the positive charge from protonated Asp side chains and the negative charge from deprotonated Asp side chains).
At pH 14 (very basic conditions), Aspartic Acid will be deprotonated (COO-) and, therefore, negatively charged. Phe remains neutral. Thus, the net charge on Phe-Asp at pH 14 would be -1.
In summary:
a. pH 1: +1
b. pH 7: -0.5
c. pH 14: -1